Shot Point Dithering Techniques for Marine Seismic Surveys

Abstract

Techniques are disclosed relating to performing marine surveys according to dither values generated based on one or more dithering constraints. This may include for example, determining a set of nominal shot points for a marine seismic energy source and determining dither values for ones of the nominal shot points. In some embodiments, the dither values are randomly generated, subject to a duplication constraint such that at most a threshold number of dither differences between consecutive shot points that fall within discrete ranges. In some embodiments, actual shot points are determined for the planned sail line based on application of the determined dither values to the nominal shot points. In various embodiments, the disclosed techniques may facilitate a separate de-blending procedure to separate signals from the marine seismic energy source and signals from one or more other seismic energy sources to be used for the seismic survey.

1 . A method for generating dither values corresponding to source activations to be performed during a marine seismic survey, the method comprising:
 determining a set of nominal shot points corresponding to a set of sources to be used during a portion of the survey; and   determining a set of dither values, each dither value in the set specifying a difference between one of the nominal shot points and an actual shot point to be produced by the set of sources during the survey;   wherein determining the set of dither values comprises satisfying one or more constraints chosen from the group consisting of: an absolute dither difference constraint, a non-duplication constraint, and a standard deviation constraint;   wherein each of the one or more constraints is based on differences between dither values corresponding to consecutive ones of the nominal shot points, wherein each such difference comprises a dither difference, such that:   satisfying the absolute dither difference constraint comprises ensuring that each dither difference corresponding to the set of dither values is greater than a threshold dither difference;   satisfying the non-duplication constraint comprises ensuring that, for a given set of discrete ranges of dither difference values, at most a threshold number of dither differences corresponding to the set of dither values falls within any one of the ranges; and   satisfying the standard deviation constraint comprises ensuring that the standard deviation of dither differences corresponding to the set of dither values is greater than a threshold standard deviation.   
     
     
         2 . The method of  claim 1 , wherein the set of sources consists of one source. 
     
     
         3 . The method of  claim 1 , wherein the portion of the survey consists of one sail line. 
     
     
         4 . The method of  claim 1 , wherein the dither values comprise units of distance. 
     
     
         5 . The method of  claim 1 , wherein the dither values comprise units of time. 
     
     
         6 . The method of  claim 1 , further comprising satisfying two or more of the constraints. 
     
     
         7 . The method of  claim 1 , wherein each of the discrete ranges corresponds to a 1000 ms interval. 
     
     
         8 . The method of  claim 1 , wherein each of the discrete range corresponds to a 100 ms interval. 
     
     
         9 . The method of  claim 1 , wherein each of the discrete ranges corresponds to a 25 ms interval. 
     
     
         10 . The method of  claim 1 , further comprising determining the threshold dither difference based on a threshold frequency to be emitted by the one or more sources during the survey. 
     
     
         11 . The method of  claim 10 , wherein the threshold dither difference is equal to one half of a cycle period corresponding to the threshold frequency. 
     
     
         12 . The method of  claim 1 , wherein the threshold number of dither differences that may fall into any one of the ranges is 1. 
     
     
         13 . The method of  claim 1 , wherein the dither differences are absolute values such that negative and positive dither differences are treated as having the same sign. 
     
     
         14 . The method of  claim 1 , wherein determining the set of dither values further comprises:
 A) generating one or more dither values randomly;   B) determining whether the dither values generated randomly satisfy one or more of the constraints; and   C) repeating steps A and B until the dither values generated randomly satisfy one or more of the constraints.   
     
     
         15 . The method of  claim 1 , further comprising storing the set of dither values in a tangible computer-readable medium. 
     
     
         16 . The method of  claim 1 , further comprising applying the set of dither values to the set of nominal shot points to produce a set of actual shot points for use during the survey. 
     
     
         17 . The method of  claim 16 , further comprising storing the set of actual shot points in a tangible computer-readable medium. 
     
     
         18 . A tangible computer-readable medium having instructions stored therein that, when executed by a computing device, cause the computing device to perform a method for generating dither values corresponding to source activations to be performed during a marine seismic survey, the method comprising:
 accessing a set of nominal shot points corresponding to a set of sources to be used during a portion of the survey; and   determining a set of dither values, each dither value in the set specifying a difference between one of the nominal shot points and an actual shot point to be produced by the set of sources during the survey;   wherein determining the set of dither values comprises satisfying one or more constraints chosen from the group consisting of: an absolute dither difference constraint, a non-duplication constraint, and a standard deviation constraint; and   wherein each of the one or more constraints is based on differences between dither values corresponding to consecutive ones of the nominal shot points, wherein each such difference comprises a dither difference, such that:   satisfying the absolute dither difference constraint comprises ensuring that each dither difference corresponding to the set of dither values is greater than a threshold dither difference;   satisfying the non-duplication constraint comprises ensuring that, for a given set of discrete ranges of dither difference values, at most a threshold number of dither differences corresponding to the set of dither values falls within any one of the ranges; and   satisfying the standard deviation constraint comprises ensuring that the standard deviation of dither differences corresponding to the set of dither values is greater than a threshold standard deviation.   
     
     
         19 . A method for performing a marine seismic survey, comprising:
 activating one or more sources at a plurality of actual shot points, wherein each of the actual shot points differs from a corresponding nominal shot point by a dither value;   wherein differences between dither values corresponding to consecutive ones of the nominal shot points constitute dither differences; and   wherein a set of dither values corresponding to a portion of the survey satisfies one or more constraints chosen from the group consisting of: an absolute dither difference constraint, a non-duplication constraint, and a standard deviation constraint; such that:   satisfying the absolute dither difference constraint comprises ensuring that each dither difference corresponding to the set of dither values is greater than a threshold dither difference;   satisfying the non-duplication constraint comprises ensuring that, for a given set of discrete ranges of dither difference values, at most a threshold number of dither differences corresponding to the set of dither values falls within any one of the ranges; and   satisfying the standard deviation constraint comprises ensuring that the standard deviation of dither differences corresponding to the set of dither values is greater than a threshold standard deviation.   
     
     
         20 . The method of  claim 19 , further comprising:
 recording signals registered by geophysical sensors responsive to activations of the one or more sources; and   storing the signals in a tangible computer-readable medium, thereby completing the manufacture of a geophysical data product.   
     
     
         21 . A non-transitory computer-readable medium having instructions stored therein that, when executed by a computing device on a marine seismic survey vessel, cause equipment associated with the vessel to perform a marine seismic survey, the survey comprising:
 activating one or more sources at a plurality of actual shot points, wherein each of the actual shot points differs from a corresponding nominal shot point by a dither value;   wherein differences between dither values corresponding to consecutive ones of the nominal shot points constitute dither differences; and   wherein a set of dither values corresponding to a portion of the survey satisfies one or more constraints chosen from the group consisting of: an absolute dither difference constraint, a non-duplication constraint, and a standard deviation constraint; such that:   satisfying the absolute dither difference constraint comprises ensuring that each dither difference corresponding to the set of dither values is greater than a threshold dither difference;   satisfying the non-duplication constraint comprises ensuring that, for a given set of discrete ranges of dither difference values, at most a threshold number of dither differences corresponding to the set of dither values falls within any one of the ranges; and   satisfying the standard deviation constraint comprises ensuring that the standard deviation of dither differences corresponding to the set of dither values is greater than a threshold standard deviation.   
     
     
         22 . The method of  claim 1 :
 wherein determining the set of dither values comprises satisfying all three of the constraints.   
     
     
         23 . The method of  claim 22 :
 wherein the portion of the survey comprises one sail line.   
     
     
         24 . The medium of  claim 18 :
 wherein determining the set of dither values comprises satisfying all three of the constraints.   
     
     
         25 . The medium of  claim 24 :
 wherein the portion of the survey comprises one sail line.   
     
     
         26 . The method of  claim 20 :
 wherein the set of dither values satisfies all three of the constraints.   
     
     
         27 . The medium of  claim 26 :
 wherein the portion of the survey comprises one sail line.   
     
     
         28 . The medium of  claim 21 :
 wherein the set of dither values satisfies all three of the constraints.   
     
     
         29 . The medium of  claim 28 :
 wherein the portion of the survey comprises one sail line.